Synthesis gas containing hydrogen and carbon oxides is an important feedstock for the production of a wide range of fuel and chemical products. For example, synthesis gas mixtures can be reacted catalytically to produce liquid hydrocarbons (e.g. via Fischer-Tropsch reactions) and oxygenated organic compounds including methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates. High purity hydrogen and carbon monoxide can also be produced from synthesis gas by further processing and/or separation of synthesis gas. Processes for the utilization of synthesis gas are well-known.
Staged steam-methane reforming (SMR) processes are used for the design of new production facilities and to upgrade the performance of existing production facilities for producing synthesis gas. One type of staged reforming process utilizes a pre-reformer (e.g., an adiabatic reforming reactor containing highly active nickel catalyst) to reform heavier hydrocarbons in the feedstock (and a portion of the methane, if present) to yield a pre-reformed mixture of methane, hydrogen, carbon monoxide, carbon dioxide, and steam. This pre-reformed mixture is then further processed in a fired tubular reformer to produce a raw synthesis gas product. Another type of staged reformer process utilizes a pre-reformer followed by an autothermal reformer (ATR). Another type of staged reformer process utilizes a gas heated reformer (GHR) followed by an autothermal reformer (ATR). The GHR is a type of heat exchange reformer in which the hot raw synthesis gas from the ATR furnishes heat for the first reforming stage in the GHR. These, and other staged synthesis gas production processes, are well-known.
Another technology for synthesis gas production involves ion transport membrane (ITM) oxidation reactors, in which oxygen ions permeate through membranes and are reacted with oxidizable compounds to form oxidized or partially-oxidized reaction products. The practical application of these oxidation reactor systems requires membrane assemblies having large surface areas, flow passages to contact oxidant feed gas with the oxidant sides of the membranes, flow passages to contact reactant feed gas with the reactant sides of the membranes, and flow passages to withdraw product gas from the permeate sides of the membranes. These membrane assemblies may comprise a large number of individual membranes arranged and assembled into modules having appropriate gas flow piping to introduce feed gases into the modules and withdraw product gas from the modules.
ITMs may be fabricated in either planar or tubular configurations. In the planar configuration, multiple flat ceramic plates are fabricated and assembled into stacks or modules having piping means to pass oxidant feed gas and reactant feed gas over the planar membranes and to withdraw product gas from the permeate side of the planar membranes. In tubular configurations, multiple ceramic tubes may be arranged in bayonet or shell-and-tube configurations with appropriate tube sheet assemblies to isolate the oxidant and reactant sides of the multiple tubes. The individual membranes used in planar or tubular module configurations typically comprise very thin layers of active membrane material supported on material having large pores or channels that allow gas flow to and from the surfaces of the active membrane layers.
Synthesis gas production systems can utilize ITM oxidation reactors in combination with other reforming techniques, such as indirect-heated primary reforming (e.g., SMR). For example, U.S. Pat. No. 6,048,472 to Nataraj et al., which is hereby incorporated in its entirety by reference, discloses a synthesis gas production system involving an initial SMR reforming step followed by final conversion to synthesis gas in an ITM oxidation reactor. The Nataraj system is designed to convert heavier hydrocarbons (e.g. C2 through C6, and greater) and a portion of feed methane to synthesis gas in the SMR, and then perform higher-temperature conversion of a portion of the remaining methane into synthesis gas in the ITM oxidation reactor. By converting the heavier hydrocarbons and generating hydrogen in the gas mixture prior to entering the ITM oxidation reactor, the ITM oxidation reactor can be operated at higher temperatures without resulting carbon deposition therein. The addition of an ITM oxidation reactor following an SMR is also an attractive option to de-bottleneck (or increase the production capacity of) an existing SMR facility.
A recent development in ITM oxidation reactor technology involves the use of a plurality of oxidation reactor stages in series. As disclosed in U.S. Pat. No. 8,262,755 to Repasky et al., which is hereby incorporated herein in its entirety, it was recognized that the exothermic reactions that occur throughout traditional ITM oxidation reactors (e.g., the ITM oxidation reactor of Nataraj) can result in excessive temperature gradients across the membranes and significantly limit membrane life, particularly in concentrated systems and/or high conversion systems (both of which are of important industrial interest). By utilizing a plurality of oxidation reactor stages in series, the extent of reaction across each stage could be regulated to limit excessive temperature gradients across the membranes of each individual stage as well as the cumulative temperature increase across the stages of the reactor system.
Such staged ITM oxidation reactor systems can be costly and difficult to construct. For example, specialized and expensive materials must be employed in the reactant zones of each stage in order to provide the inter-stage injections, such as special injector nozzles that can reliably withstand elevated temperature and pressure conditions.
In commercial synthesis gas production environments that utilize ITM oxidation reactor systems, including known staged and non-staged ITM reactor systems in combination with other technologies such as oxygen-blown primary reforming (e.g., ATR) and/or indirect-heated primary reforming (e.g., SMR), the performance of the ITM reactor systems is sensitive to changes in system operating conditions and degradation of the membrane material performance over extended time of operation (or age of the membrane materials in operation). For example, the ceramic membrane material and the components of the membrane modules can be subjected to significant mechanical stresses during normal steady-state operation and during unsteady-state startup, shutdown, and upset conditions. These stresses may be caused by thermal expansion and contraction of the ceramic material and by dimensional variance caused by chemical composition or crystal structure changes due to changes in the oxygen stoichiometry of the membrane material. In addition, membrane materials have upper temperature limits above which membrane material degradation and/or module damage may occur. All of these effects can negatively impact the performance of the ITM oxidation reactor system and therefore the larger synthesis gas production process. Even under ideal operating conditions, some degradation in membrane performance is to be expected over extended time of operation.
There is a need in the art for synthesis gas production systems comprising ITM oxidation reactors and methods for operating the same that are cost-effective and which enable operators to readily compensate for membrane performance degradation and other changes in system operating conditions that affect synthesis gas production.